In a fibre-optic network, the wavelength of light used to communicate a signal is an important parameter. In particular, where Wavelength Division Multiplexing (WDM) systems are employed, different signals are communicated using different wavelengths. Consequently, it is important to maintain the different wavelengths accurately in relation to components of the WDM system, for example multiplexers and demultiplexers, which add or remove wavelengths from the WDM system.
Typically, a semiconductor laser device is employed in a transmitter unit of the fibre-optic network. The wavelength of light transmitted by the laser device can be accurately controlled by altering a parameter, for example temperature, of the laser device using a closed loop feedback circuit. In this example, in order to determine whether to heat or cool the laser device, and to what extent, to restore the wavelength of the laser device to a predetermined wavelength, an error is typically generated in the feedback circuit.
Known apparatus for detecting changes in the wavelength of a laser device are disclosed inter alia, in U.S. Pat. Nos. 4,309,671, 6,144,025, 5,825,792, all of which are incorporated herein by reference.
U.S. Pat. No. 4,309,671 discloses a laser diode, a proximal beam splitting mirror and a proximal photodiode to receive light from the proximal beam splitting mirror, a distal beam splitting mirror and a distal photodiode to receive light from the distal beam splitting mirror, and a filter disposed between the distal beam splitting mirror and the distal photodiode. An electronic control circuit stabilizes the laser diode. In use, a divergent beam is emitted by the laser diode. The proximal beam splitting mirror directs a portion of the light incident on the proximal beam splitting mirror onto the proximal photodiode. Similarly, light passing through the proximal beam splitting mirror is incident on the distal beam splitting mirror. The distal beam splitting mirror directs a portion of the light incident on the distal beam splitting mirror onto the distal photodiode. Since the light continues to diverge as it propagates from the proximal beam splitting mirror to the distal beam splitting mirror, the distal photodiode only receives a small fraction of the light directed towards the distal photodiode. In order to ensure that beams eminating from the proximal and distal beam splitting mirrors are not obstructed, the beam splitting mirrors and the photodiodes must be widely spaced. The provision of two beam splitting mirrors spaced apart and the need for individual placement and careful alignment of the beam splitting mirror results in relatively high cost and large volume of the above apparatus. Similarly, the need to carefully place and align the proximal and distal photodiodes is a costly exercise that contributes to the large volume of the apparatus. Additionally, the spacing of two beam splitting mirrors results in the proximal and distal photodiodes being unable to make efficient use of the portions of light respectively directed towards them due to the divergent nature of the beam emitted by the laser diode.
U.S. Pat. No. 5,825,792 discloses a relatively compact apparatus comprising a lens, a Fabry-Perot etalon and two photodiodes. The apparatus is small enough to be copackaged with a semiconductor laser in an industry standard package known as a “butterfly” package. The etalon splits light emitted by the semiconductor laser and directs the light over multiple paths of different lengths before recombination. Respective wavelength dependent phases are accumulated over the multiple paths. Consequently, the result of the recombination also depends on wavelength.
The dimensions of the etalon depend on a required resolving power, R, of the etalon; the resolving power is a measure of a minimum change of wavelength that can be detected. The resolving power, R, of the etalon is given by the following equation:
  R  =      F    ⁢                  2        ⁢        nd                    λ        o            where:                F is the coefficient of finesse,        n is the refractive index of the etalon,        d is the thickness of the etalon, and        λo is the wavelength of operation.        
As a practical example, in order to monitor a 100 GHz or 50 GHz channel spacing, at least one dimension of the etalon has to be approximately 1 mm or approximately 2 mm, respectively. Clearly, such dimensions are large compared with a typical dimension of a semiconductor laser of approximately 300 μm.
Hence, as explained above in relation to U.S. Pat. Nos. 4,309,671 and 5,825,792, to copackage the semiconductor laser device with the etalon requires a package that is substantially larger than a package for the semiconductor laser device alone, since for small channel spacings the dimensions of the etalon are large. Also, the etalon of the apparatus of U.S. Pat. No. 5,825,792 requires very precise angular alignment with respect to the beam emitted by the laser device. Furthermore, with respect to U.S. Pat. No. 5,825,792, no “transmitted beam” is provided for onward propagation into a WDM system. There is therefore a need to locate the apparatus of U.S. Pat. No. 5,825,792 adjacent a back facet of the semiconductor laser, thereby restricting available space for other components as well as, in some cases, disadvantageously increasing lengths of Radio Frequency (RF) paths to the semiconductor laser.
U.S. Pat. No. 6,144,025 discloses a laser diode coupled to a first optical fibre. In use, light emitted by the laser diode propagates through the first optical fibre, a lens, a cut filter, thence the light is incident upon a beam splitter. A first photodiode is located on a first side of the beam splitter and a second photodiode is located on a second side of the beam splitter. An optical band-pass filter is disposed in-line between the beam splitter and the first photodiode. A portion of the light incident on the beam splitter is directed towards the first photodiode. A first portion of the light directed towards the first photodiode is passed through to the first photodiode and a second portion of the light directed towards the first photodiode is reflected by the optical band-pass filter and coupled to the second photodiode via the beam splitter. A certain portion of the light incident on the beam splitter via the cut filter passes directly through the beam splitter to a lens that focuses the transmitted light into a second optical fibre.
The apparatus of U.S. Pat. No. 6,144,025 requires the first and second photodiodes to be relatively widely separated. The photodiodes can not, therefore, be formed as a joined pair of detectors and require individual placement and alignment. Also, the beam splitter and the optical band-pass filter have to be aligned with angular precision, because light incident on the second photodiode is reflected by the beam splitter and the optical band-pass filter. Small angular errors in the position of the beam splitter and the optical band-pass filter cause the beam to be displaced laterally at the locations of the first and second photodiodes. Additionally, the first and second photodiodes of the apparatus of U.S. Pat. No. 6,144,025 are separate and so are susceptible to the effects of ageing and temperature differences.